1. Field of the Invention
The present invention relates to a ZnO-based semiconductor light emitting element and to a method for manufacturing the same.
2. Description of the Related Art
ZnO-based semiconductor light emitting elements have advantages over other semiconductor light emitting elements, such as having a larger exciton binding energy, using lower-cost raw materials, and being less harmful to people and the environment. Therefore, such ZnO-based semiconductor light emitting elements are expected to provide high-efficiency and low-power consumption devices.
Examples of the substrates used in ZnO-based semiconductor light emitting elements include zinc oxide (ZnO) substrates, sapphire (α-Al2O3) substrates, and silicon carbide (SiC) substrates. Of these, sapphire substrates are preferred because they cost less than zinc oxide and silicon carbide substrates.
In one known method for manufacturing a ZnO-based semiconductor light emitting element using a sapphire substrate, a single crystal of α-sapphire is used. A single crystal of α-sapphire has a hexagonal crystal structure. Three representative types of substrates cut from such a single crystal are a sapphire C-plane substrate having a principal surface lying in the C-plane ({0001} plane), a sapphire A-plane substrate having a principal surface lying in the A-plane ({11-20} plane), and a sapphire R-plane substrate having a principal surface lying in the R-plane ({10-12} plane).
The ZnO-based semiconductor light emitting element includes a ZnO-based semiconductor layer formed on the substrate. The ZnO-based semiconductor layer is formed by epitaxial growth of ZnO crystals on the substrate and serves as an operating layer composed of, for example, an n-type ZnO layer, a light emitting layer, and a p-type ZnO layer.
When the ZnO-based semiconductor layer is formed on a sapphire substrate, the properties of the grown ZnO crystal vary depending on the principal surface of the sapphire substrate where the layer is formed. When a sapphire C-plane substrate is used, the grown ZnO crystal contains 30° rotated domains, and therefore the crystallinity is poor. When a sapphire R-plane substrate is used, the grown ZnO crystal has streak-like projections on its surface and is not suitable for device formation. When a sapphire A-plane substrate is used, the grown ZnO crystal does not contain 30° rotated domains, and a flat surface can be obtained. Therefore, a sapphire A-plane substrate is most suitably used to produce ZnO-based semiconductor light emitting elements.
Generally, the case, frame, stem, and other components used to mount a semiconductor light emitting element are designed based on the premise that the semiconductor light emitting element has a rectangular shape. Therefore, a rectangular shape is most suitable for semiconductor light emitting elements manufactured by dividing a substrate having a semiconductor layer formed thereon.
The following method is known as a method for dividing a sapphire A-plane substrate having a GaN-based semiconductor layer formed thereon into rectangular pieces. That is, in Japanese Patent Application Laid-Open No. 2003-086541 (corresponding to US 2005/0003632A1), a sapphire A-plane substrate is prepared which has, for example, a diameter of 50.8 mm (2 inches) and a thickness of 300 to 500 μm. The sapphire A-plane substrate includes a GaN-based semiconductor layer formed on a first side. Then, first scribed grooves L1 and second scribed grooves L2 orthogonal to the first scribed grooves L1 are formed on a second side or c-m plane of the sapphire A-plane substrate that is opposite to the first side and is formed by the c-axis and the m-axis orthogonal to the c-axis. In this instance, the first scribed grooves L1 form an angle γ of 47.7 to 51.7° with respect to the c-axis. The second scribed grooves L2 are formed to be orthogonal to the first scribed grooves L1. Next, the sapphire A-plane substrate is broken along the first scribed grooves L1 and then along the second scribed grooves L2 to divide the sapphire A-plane substrate into separate sapphire A-plane substrates. The conventional method disclosed in this publication can form separate rectangular sapphire A-plane substrates so that GaN-based semiconductor light emitting elements can be manufactured with a yield of 96 to 98%.
The present inventors have attempted to apply the method disclosed in Japanese Patent Application Laid-Open No. 2003-086541 to a sapphire A-plane substrate having a ZnO-based semiconductor layer formed thereon. However, the inventors have found that cleavage planes R1 and R2 (thick lines in the figure) deviate substantially from the scribed grooves L1 and L2, as shown in FIG. 1. Therefore when the method disclosed in this patent publication is applied to a sapphire A-plane substrate having a ZnO-based semiconductor layer formed thereon, ZnO-based semiconductor light emitting elements including rectangularly divided sapphire A-plane substrates cannot be manufactured.
Generally, when semiconductor light emitting elements are manufactured, a substrate having a thickness of 300 to 500 μm is ground and polished to a thickness of approximately 200 μm or less and is then cleaved, as described above. If the substrate itself is used for a crystal growth test or an element formation test, the substrate having the original thickness (300 to 500 μm) is often used. In such a test case, the relative depth of the scribed grooves with respect to the thickness of the substrate is smaller than that when the semiconductor light emitting elements are manufactured, and therefore the substrate is more difficult to cleave. The depth of the scribed grooves is generally about 2 to about 6 μm, depending on the hardness of the substrate. The ease of cleavage is not determined only by the depth of the scribed grooves. However, the larger the relative thickness of the substrate with respect to the depth of the scribed grooves, i.e., the smaller the relative depth of the scribed grooves, the more difficult to cleave the substrate. For example, if the depth of scribed grooves is 3 μm, the relative depth of the scribed grooves with respect to a 50 μm thick substrate is 6%, and the relative depth of the scribed grooves with respect to a 100 μm thick substrate is 3%. The relative depth of the scribed grooves with respect to a 200 μm thick substrate is 1.5%, and the relative depth of the scribed grooves with respect to a 430 μm thick substrate is 0.7%.
Therefore, it is desirable to form rectangularly divided sapphire A-plane substrates using not only a sapphire A-plane substrate having a thickness of approximately 200 μm or less but also a sapphire A-plane substrate having a thickness of 300 to 500 μm.
In a semiconductor light emitting element, the light emitted from the light emitting layer is emitted to the outside or attenuated inside the semiconductor light emitting element through one of the following three processes.
1) The light passes through the inside of the semiconductor light emitting element and is emitted to the outside.
2) The light is reflected and scattered inside the semiconductor light emitting element and then emitted to the outside. In this case, part of the light is absorbed by an electrode and the crystal that forms the semiconductor light emitting element.
3) The light is absorbed and attenuated inside the semiconductor light emitting element.
Therefore, to increase the light emitting power of the semiconductor light emitting element, it is necessary to increase the ratio of light emitted to the outside in process 1), to reduce the frequency of reflection and scattering in process 2) to increase the ratio of light emitted to the outside, and to reduce the ratio of light absorbed and attenuated inside the semiconductor light emitting element in process 3).
In one light-extraction technique used to increase the light emitting power of a semiconductor light emitting element, the aspect ratio of the semiconductor light emitting element (the ratio of the side length of the semiconductor light emitting element to the height thereof) can be reduced, i.e., the height of the semiconductor light emitting element can be increased. By increasing the height of the semiconductor light emitting element, the area of the side surfaces thereof is increased, and therefore the amount of light emitted from the side surfaces is increased, so that the light emitting power can be increased.